Fire warning systems



Oct. 15, 1968 J. C. NAILEN 3,406,389

I FIRE WARNING SYSTEMS Filed Aug. 23. 1965 3 Sheets-Sheet l d PO -i LL.9

2| F I I- I I I g; I I I I I I I. I v Q c s I 2 8 5 I 1? I I M m I N I NI 3 a I I I O I I MMHJ L I g a I I I II w F a I 3 N I I N I qh I I o alI I James C. Noilen, I INVENTOR. BY. L W M- u ATTORNEY.

J. C. NAILEN FIRE WARNING SYSTEMS Oct. 15, 1968 3 Sheets-Sheet 3 FiledAug. 23, 1965 INVENTOR.

.ATTOR .l

Un'itecl States Patent 3,406,389 FIRE WARNING SYSTEMS James C. Nailen,Santa Ana, Calif., assignor, by mesne assignments, to WhittakerCorporation, Los Angeles, Calif., a corporation of California Filed Aug.23, 1965, Ser. No. 481,849 17 Claims. (Cl. 340-411) ABSTRACT OF THEDISCLOSURE A fire warning system for use with heat sensors of the typewherein a heat sensitive salt insulates coaxial conductors under normalconditions and reduces in resistance when heated. The systemdiscriminates between the relatively slow resistance decreaserepresenting heat, and also the rapid resistance drop of short circuitproducing faults in the salt, and actuates an appropriate alarm. If thesystem senses a short circuit, it applies a plurality of electricalpulses across the salt which, in most instances, will cure the fault andrestore the heat sensor to an operational condition.

This invention relates in general to fire warning systems and relates inparticular to fire warning systems for aircraft.

More specifically, this invention is an improvement in aircraft firewarning systems so that the crew is reliably informed when the aircraftis, in fact, on fire or that an area is overheating; that this warningis trustworthy and can be relied upon; and that this warning is not afalse alarm. In order to accomplish this, the invention is able todetect and, in almost all cases, cure the system of defects which cause,by far, the largest number of false alarms.

In addition to the incorporation of reliability and certitude intoexisting fire warning systems, this invention also includes otherimprovements in existing fire warning systems to give the crew stillmore information, such as, malfunctioning of the system due to agrounded wire; but first, in order to understand the problem solved bythis invention and how this invention works, an explanation of one typeof existing fire warning systems is in order.

The existing fire warning systems in aircraft comprise one or more heatsensing units or sensors located where overheating or fire might beexpected to occur, for example, they are located adjacent to or wrappedaround each engine, or located in wheel well areas, or in any otherlocation which has combustibles fed thereto. These heat sensors vary inlength and are connected in series with a pilot test button and a firewarning light both of which are conventionally located in the cockpit insuch a position that the pilot and/0r engineer can cut the flow ofcombustibles to the area indicated by the lighting of the fire warninglight to reduce the danger to the aircraft. The pilot test buttonpermits the crew to test the continuity of this system before the flightor during flight in order to determine whether the system is inoperation.

Thus, in the existing systems, the crew members have two bits ofinformation fed to them: (I) the system is in operation (by depressingthe pilot test button to light the fire warning light) and (2) the firewarning light which indicates (theoretically) that there is fire and/oroverheating in some particular area.

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Under law, if a fire warning light becomes lit and the aircraft is onthe ground, the pilot must not take off or, if in flight, must make aturn around or seek the nearest adequate airport and abort the flight toavoid the possibility of a disaster.

Unfortunately, however, the existing fire warning systems are subject toa serious defect. This defect is the fact that these systems will oftenindicate a fire when, in fact, none exists and thus give out a. falsealarm. Unfortunately, too, experience has shown that about 90% of thefire warnings have been false alarms with the result that flightabortions and the cost thereof to the airlines are unduly high. Forexample, it is reliably estimated that the cost of each flight abortionis between $10,000 and $25,- 000 to say nothing of passenger ill-willincurred toward the airline due to the inconvenience caused.

Thus, when it is realized that about 90% of the total cost of flightabortions in any given period is unnecessary simply because present firewarning systems give out an excessively high number of false alarms, itcan be appreciated that a solution to this problem becomes quiteparamount. It should also be borne in mind that when the pilot isrequired by law to seek the nearest airport, he is required to landunscheduled at an unfamiliar field which, itself, is dangerous. Equallyas important, however, is the fact that, since 90% of the fire warningsare false alarms, pilots, knowing this, will unfortunately tend toignore or become complacent about the fire Warning light.

Numerous attempts in the past have been made to solve this problem ofunnecessary high incidence of false alarms in fire warning systems andthe typical solution suggested is the use of alternate circuits betweenthe heat sensors and the fire warning light and coupled in such a mannerthat at least two of such circuits must be in operation before the firewarning light will go on. This system assumes, of course, that the heatsensor will short both circuits and thereby light the fire warning lightand assumes that it is unlikely that shorts, such as grounded wires, ina number of circuits will occur at the same time. This system, ofcourse, does increase the reliability of fire warning systems somewhatbut does not really solve the problem of false alarms because experiencehas shown that a short due to the grounding of a wire in the circuitaccounted for only about 1% of the total number false alarms; theremaining number (89%90%) of false alarms is due to a fault, i.e., adefect or short, occurring in the sensor itself which cannot bedistinguished by this system. In other words, the suggested system, atbest, would only decrease the number of false alarms by about 1%.

The present invention, on the other hand, not only will detect a shortdue to a fault which has occurred in the heat sensor and which wouldnormally cause the fire warning light to go on, but will attempt to andwill (in nearly all cases) cure this fault, and will do thisautomatically before the fire'warning light now has a chance to go onthereby not alarming the crew. If unable to cure pect of this invention,the reliability of fire warning sys-. terns is increased by theelimination of 90% of the fire warnings which have false alarms leavingonly about 1% of the fire warnings due to false alarms due to shortcircuits, such as grounded wires, to be detected. These latter falsealarms cannot be cured by this aspect of the invention, but thisinvention does include a detector which identifies these shorts andadvises the crew that there is such a short in the system and that theycan no longer rely on this system to detect a fire. As a matter of fact,with this invention not only is there a warning of a short in thesystem, but the fire warning light is prevented from lighting when thisoccurs so that the crew will not mistake the fact that a short exists inthe system and the crew can no longer rely on the system to warn of afire.

Now, in order to understand how the present invention solves theaforementioned problems and overcomes the deficiencies of the presentfire warning systems and obsoletes the numerous attempts in the past tosolve these problems, an explanation of the existing systems, andparticularly the heat sensors in more detail is in order at this time.

A heat sensor comprises a hollow cylindrical tube or shell with acoaxial conductor separated from the wall of the outer tube by a salt.These sensors can vary in length from about three feet to 25 feet andhave a diameter of about 0.1 inch with the radial distance from thecenter conductor to the wall of the outer tube being about 0.02 inch.The shell is usually made of Inconel, a very high temperaturenickel-iron alloy, and the center conductor is nickel.

The salt between the center conductor is eutectic in that it is acompound designed to undergo a structural change at some predeterminedtemperature and is of a type chosen to electrically insulate the centerconductor from the outer shell at low temperatures, and drop sharply inresistance, i.e., conduct current at some predetermined highertemperature. Thus, when the center conductor is connected to a source ofcurrent and an impedance measuring device, any drop in resistance due tothe heating of the sensor caused by the salt losing its ability toinsulate the center conductor from the outer shell will be detected orsensed by the impedance measuring device. In the present fire warningsystems, the detection or sensing of a drop in resistance is visuallyreferenced for the pilot by the energization of the aforementioned firewarning light coupled into the circuit.

Thus, as can be appreciated, much depends on the salt in the heat sensorfor the correct operation of existing fire warning systems andheretofore the real problem of false alarms, namely, the malfunctioningof the salt was left unsolved. It was discovered, however, that if smallamounts of electrical energy (usually one to three pulses of suchenergy) are applied to the center conductor, the defects or faults canbe removed from the sensor. It was also found that in nearly all casessuch faults were cured permanently.

The discovery and cure of such faults forms a major aspect of thisinvention, as previously stated, and the 'following is an explanation asto why such faults can be cured in the manner taught by this invention.

It is reasoned that if the salt is pure in form and voidof any defects,no conduction will occur from the center conductor to the outer shellunless heat is applied to the sensor. It is also reasoned that the salthas a very definite lattice structure and in such lattice structurethere are a number of defects which may occur and which will affect theinsulative properties thereof, because the current carr'iers in thecrystalline structure (i.e., electrons, protons,

cations, anions, or any combination thereof) are affected.

These defects are well known; for example, there are' (l) Frenkeldefects where either a cation or anion is removed from its interstitialposition, to a point where it can no longerreturn to that positionleaving a hole in the lattice structure and a free ion, (2) Schottkydefects where both the anion and the cation lattice points are deserted,thus preserving the electrical neutrality of the crystalline interior,(3) planar slippage of the interstitial ions of the lattice structure,and (4) a dendr1t1c formulation of the lattice interstitial ionalteration.

Of these defects, the Frenkel and Schottky defects can be due totemperature, impurities, voltage gradients, and extreme sonic vibration.The other defects are also due to the same factors plus a mechanicalstress such as caused by the difference in the coefficient of expansionof the material used in the sensors upon the application of heatthereto, or by mechanical shocks such as vibrations or twistin of thesensors.

Thefefore, with dendritic and planar faults created by physical stressesand mechanical shocks, 1t becomes clear that heating and cooling of thesensor as well as twisting, etc., during installation could create suchfaults. It is even suspected that some faults may be created during themanufacture of the sensor and which are subsequently cleared but whichare reintroduced during the operation of the aircraft by reason ofvibration. It seems also clear that the heating or the cooling of thewhole sensor as a complete body is of no avail since the same faultsWlll reoccur when the sensor is heated or is cooled as the case may be.This also may explain why it is difficult to determine the cause of afalse alarm because the fault maydisappear when the sensor is cooled. f

Accordingly, since these dendritic faults conslst o fern-like formationsprotruding from the inner .center conductor and from the inner wall ofthe outer shell through the crystal thus causing current circuits, andsince the unstable energy state caused by these faults are extremelyminute, they are disturbed by very small amounts of external energyappropr ately applied, that is, applied only through the fault region,leaving the surrounding crystalline structure undisturbed.

It is believed, therefore, that by passing a sufficient electricalcurrent through the center conductor to cause an Internal thermalheating sufficient to overcome the valence bonds of the latticestructure, the crystal is allowed. to melt and resolidify into itslowest energy state which 1s that of the pure crystalline latticestructure. Since the pure crystalline lattice structure precludes theexistence of free ions, electrons and other current earners, the crystalis once again a perfect dielectric and insulator.

It was found that the energy required to cure these faults are rathersmall which also bears out the above theoretical explanation of thesefault shorts. In fact, it was determined that in most cases the faultcan be cured by discharging 20 volts from a 0.5 mfd. capacltor, and in afew stubborn cases a discharge of volts from a 1 mfd. capacitor wasrequired. Furthermore, it was found that the energy introduced into thesensor was so minute that it did not disturb the balance of thecrystalline structure or other adjacent electrical devices, and the moretimes the sensor was cleared, the harder it became to make a faultoccur. Thus, an essentially faultless sensor.may be obtained afterseveral clearings.

Thus, from the foregoing, it can be seen that the high occurrence offalse alarms can be eliminated by the teachings of this invention by theintroduction of a clearing or curing pulse through the sensor when theshort occurs and is reflected into the remainder of the fire warningsystem and it is to this end that this invention is primarily directed.

In addition to the foregoing principal object of this invention beingaccomplished, it is still another object of this invention to provide asystem by which the decrease in resistance due to a fault occurring inthe sensor may be sensed or detected and the sensor pulsed by the supplyof DC voltage to cure the fault. This is accomplished in the" presentinvention by providing a circuit which is responsive' to the decrease inresistance in the sensor and which will act to pulse the sensor with thesupply of DC voltage.

Furthermore, in order to make the existing fire warning paths or short.

systems more reliable and to give the crew still more in formation, thisinvention further includes a means by which true short circuits, thatis, shorts occurring by reason of a grounded wire or the like, may bedetected and the crew advised thereof.

As a further refinement to the existing fire warning systems, thisinvention further includes means which enable the crew to rely on thefact that when they are advised of a short existing in the system, thereis in fact a short and the system can no longer detect a fire. In otherwords, with this invention, the crew can be reliably informed that afire actually exists or, alternatively, that a short has occurred in thesystem which is not due to a fault in the sensor.

Other additional features and advantages of this invention will becomeapparent to those skilled in the art from the following specificationsand drawings which form a part thereof and wherein:

, FIG. 1 is a schematic illustration of a present fire warning system;

FIG. 2 illustrates the present invention schematically in block diagramsand incorporated in, and forming part of, the present fire warningsystem of FIG. 1;

FIGS. 3 and 3a are schematic illustrations of the circuit involved inthe block diagrams of FIGS. 1 and 2; and

FIG. 4 is a graph of the temperature-resistance and time relationship ina fire warning system.

The present fire warning system is shown in FIG. 1, in block diagramsand separated in FIG. 2 to illustrate clearly how this invention may besimply coupled into (and form part of) the present fire warning systemwithout interference to the latters performance or operation.

Turning now specifically to FIG. 1, it can be seen that the existingsystem comprises a plurality of heat sensors; one being illustratedherein simply for the purpose of describing the system and designated as10. This heat sensor comprises outer metallic tube or shell 12 having acoaxial center conductor 14 separated by a heat sensitive insulativematerial, usually eutectic salt, which at low temperatures electricallyinsulates the center conductor 14 from the shell 12 and thus acts like avery high resistance, almost an open circuit at the current level ofthis system, but which at elevated temperatures will lose its highresistance and conduct current therethrough substantially shorting thecenter conductor 14 to ground through tube 12. The existing system isalso provided with a detector comprising in this embodiment a lowimpedance generator or a magnetic amplifier 18 connected between thecenter conductor 14 of the sensor 10 by a main sensing line 20 and afire warning light 22. The detector loop for the fire warning systemcomprises magnetic amplifier 18, which applies AC current through line20 and line 24 and to both ends of center conductor 14; contact 26 of anormally closed single pole double throw pilot test relay 28, beingclosed at this time. The coil 30 of the pilot test relay 28 is coupledto a pilot test switch 32 which in turn is connected to the ships supplyof DC voltage (+28 v.). The closing of the pilot test switch 32 throwsthe relay swinger 34 to contact 36 which connects line 24 to groundenergizing fire warning light 22.

In the existing system, as herebefore mentioned, the fire warning light22 and the pilot test switch 32 are located in the cockpit adjacent thehandle connection to the supply of all combustibles fed to each of theengines and to the landing gear. Usually, the system also has a buzzerconnected in series with the fire warning light 22 to call the crewsattention to the fact that the fire warning light is on.

Under normal condition, the magnetic amplifier 18, supplying AC currentthrough the main sensing line 20 and sensing the reflected impedancetherein will not energize the fire warning light 22, but normalprocedure calls for the pilot to test the system by closing pilot testswitch 32 to close contact 36 and thus ground the magnetic amplifier 18which will, of course, temporarily energize fire warning light 22.However, while testing the continuity of the system, this test assumesthat the magnetic amplifier 18 is working normally, that none of theconnections between the various sensors are broken and the test isindicative that the center conductor 14 is unbroken. Nonetheless, sincethe sensing line 20 is shorted to ground (thus energizing the firewarning light 22), a short to ground, a short in relay 28, or a firecannot be distinguished. This means that, if the fire warning light 22were to go on at any time, the pilot cannot tell by this continuity testwhether the plane is in fact on fire or whether the short is caused bysome other means. Unfortunately, since sensors often develop faults orshorts, as discussed previously, which are reflected in the magneticamplifier 18 and in the fire warning light 22 even when in fact there isno fire in the aircraft, the information available to the crew, (i.e.,fire warning light 22 and the continuity test by closing pilot switchtest 32) cannot inform them whether there is a short due to a fault inthe sensor, a short due to grounding a Wire, or a fire. Thus, asrequired by law, the pilot must either not take off, or if in flight,must make a turn around (or seek the nearest suitable airport) and aborthis flight. As hereinabove mentioned, this costs the airlines from$10,000 to $25,000 for each abort as well as much passenger ill-will andoften causes the pilot to land unscheduled at an unfamiliar field. T00,and perhaps equally as important, is the fact that since of the firewarnings are false alarms, pilots knowing the unreliability of thesystem, will unfortunately tend to be complacent about the fire Warninglight.

As hereinabove mentioned, this invention will detect and cure faults inthe sensors without affecting the performance or operability of existingsystems but necessarily incorporating more reliability into existingsystems.

How the reliability is incorporated into the existing systems (eitheralready installed in aircraft or forming part of newly installedsystems) by the disclosed embodiment oFfIinvention will now be describedin connection with In this figure, for purposes of clarity, most of theexisting system, i.e., the sensor 10 and pilot test relay 28, have beenplaced within the phantomized block diagram in the upper left handcorner in FIG. 2 above the magnetic amplifier 18.

Taking first the means for curing a short due to a fault in the sensoritself, it can be seen that this cure is accomplished in the embodimentdisclosed by applying one or more pulses of high DC voltage from aclearing supply 40 through a clearing relay 42 through the main sensingline to the center conductor 14 of the sensor 10. This pulse, or pulses,of high DC voltage returns the eutectic salt to its origlnal state ofhigh resistance, as discussed supra. (It is to be noted parentheticallyat this point that, while DC voltage is applied in this embodiment, anAC voltage could be used as, for example, from a blocking or square waveoscillator.)

Secondly, this figure shows the means to detect or sense the occurrenceof the fault so as to actuate the clearing relay 42.

This means for detecting a fault and to actuate the clearmg relay 42comprises an amplifier 50, a rectifier 52, a filter 54; the output ofwhich is connected to a clearing trlgger circuit '56. Clearing triggercircuit 56 is connected to a clearing cycle relay limiter 58. Althoughconsidered as part of the fault sensing means, the clearing cyclelimiter 58 is to limit the action of the clearing relay 42 to apredetermined number of cycles so that the sensor is pulsed not morethan, for example, 3 times should a single pulse be insutficient to curethe fault. The reason for this limitation on number of times theclearing relay 42 is cycled is to allow the continued decrease inresistance in the sensor due to heat to thereafter continue to bereflected in the system to warn of a fire or a short as the case maybe;it being known by experiment that if the resistance in the main sensingline 20 cannot be restored to its high level, i.e., the fault cured, bythree or four pulses of high DC voltage, the decrease is not due to afault but due to a fire or a short other than a fault. An inherentadvantage in the limitation of the number of cycles of operations ofthis switching relay 42 is, of course, the preservation of the life ofthe relay and thereby increase the reliability of present invention, aspart of a fire warning system. In other Words, while a decrease inresistance due to a fire, a fault, or a short (other than a fault) willbe sensed by the fault sensing means, if the decrease is caused by afire or a short, the decrease is continual and this invention willautomatically attempt to cure the decrease as if it were due to a fault;but after the sensor is pulsed the predetermined number of times, thecontinued decrease in this main sensing line cannot be cured by thisinvention. It can be appreciated, therefore, that while this inventionwill act automatically to attempt to cure a fault, it will not interfere'with a true fire indication as sensed by the sensor, nor will itinterfere with a short (a true grounding short) indicated by thisinvention to be in the system as will now be described.

It is to be understood that this short warning indication informs thecrew that a short exists in the system, which short is not due to afault in the sensor or due to a fire.

This short indicator means is illustrated in FIG. 2 and comprises ashort indicating light 60 and the means to energize and control thelight comprising the aforesaid amplifier 50 and rectifier 52, a secondfilter 62 connected to rectifier 52 and a short trigger circuit 64. Theshort trigger circuit 64 is coupled to a gate circuit 66, a shortdetector 68 and a relay driver 70 for actuating switching relay 72. Gatecircuit 66 is also coupled to a time delay 74 which prevents the shorttrigger 64 from operating the short detector 68 unless the short trigger64 is operated within a predetermined time. If the short trigger 64 isunable to operate the short detector 68 and the relay driver 70 withinthis short time delay, the magnetic amplifier will energize the firewarning light to advise the crew that a fire in fact exists.

In order to fully explain the function and operation of the clearingpulse circuit and the short light energizing control means, reference isnow directed to FIG. 4.

This figure is a graph which shows the relationship between the sensortemperature and resistance as a function of time. Curve 80 shows theresistance drop in a sensor such as heated by a relatively cool fire(700 degrees F. to 1000 degrees F.) and curve 82 shows a resistance dropin a sensor heated by a relatively hot fire (1600 degrees F. to 2400degrees F.). These curves are typical of the resistance drop to beexpected to be reflected in the main sensing line and in the magneticamplifier 18. It is to be noted that at the temperature of about 100degrees F. the resistance of the sensor, that is, the resistance of thecenter wire 14 in series with the eutectic salt to ground, is not lessthan 100 ohms and can be as high as 1 megohm and that when the sensor isfully heated to about 1000 degrees F. the resistance will be about ohmsand at about 2400 degrees F. the resistance of the sensor isapproximately 5. ohms. Since the curves 80 and 82 approach the lesserresistance asymptatically, in no event does the resistance of the sensorgo below the 5 ohm level due to a fire even under the worst conditions,that is, a fire near the very end of a sensor connected in parallel(such as shown in FIGS. 1 and 2). v

It should be noted also that with a hot fire (curve 82), the resistancedrop from maximum level to the 100-70 ohm level has a transition time offrom 100 to 200 milliseconds, but thereafter the drop in resistancebecomes less and less rapid taking about one second to reach the 30-5ohm level. The drop in resistance to the cooler fire (curve 80) takesmuch longer and a further drop in resistance can vary from about 300milliseconds to five minutes, depending upon the distance of the sensorfrom the source of radiation. The leveling off of the resistance dropbelo wthe -70 ohm level is believed to be due to re-radiation of thesensor; the hotter sensor being a white hot body. Thus, even with thehottest fire, the transition time to the lower resistance from themaximum level to the 100-70 ohm level is from 100 to 200 milliseconds,but thereafter the transition time to drop to the 5 ohm level is about300 milliseconds.

This graph also shows that about 580 degrees F. the resistance of thesensor is about 30 ohms and the transition time to that resistance levelvaries from milliseconds to 300'milliseconds or more and it is at thisresistance level that the magnetic amplifier 18 isset to light the firewarning light 22 in the existing fire Warning systems. Note in thegraph, that curve 84 representing a short will reflect a drop inresistance to the 10070 ohm level much the same as the drop in theresistance due to a very hot fire, but thereafter the resistancecontinues to drop rapidly until it reaches a level between 2 to 6 ohms.The rapid drop in resistance due to a short, as shown in this graph,will have a transition time from the 100-70 ohm level to the almost zeroresistance level of about one millisecond.

From the foregoing it can be seen that, if the sensitivity of the faultcuring sensing means to pulse the sensor to cure a fault is set at100-70 ohm level, the sensor will be pulsed in an attempt to cure thefault before the resistance will drop to the fire warning level-30 ohms.If the fault is cured, of course, the level in resistance returns to itsmaximum level but, if there is actually a fire, the resistance willcontinue to drop to some level below the 580 degree F. level. Thismeans, of course too, that the sensor will be pulsed in an attempt tocure a fault if in fact a short due, for example, to a grounded wire isreflected in the magnetic amplifier. Thereafter, whether the firewarning light is energized or whether the short indicating light isenergized, depends upon the transition time sensed by this invention.

Turning now to the problem of a short, this graph clearly shows that, ifa short has in fact occurred, the drop in resistance to the 2-6 ohmlevel will occur within 130 milliseconds; whereas, even with the hottestfire, the drop in resistance to the 25-5 ohm level requires at least 300 or more milliseconds. Therefore, if the time delay 74 and gate 64 arearranged to delay energizing of the fire warning light by the magneticamplifier for some period exceeding 130 milliseconds but less than the300 milliseconds, then the short indicating circuit time would have timeto energize the short warning indicator light 60. If the latter occurs,the fire warning light 22 would never be energized. On the other hand,remembering that the drop in resistance to the 6-2 ohm level Will occur,if at all, before 130 milliseconds, and if the gate circuit 66 and timedelay circuit 74 are arranged to prevent the energizing of the shortindicating light 60 after 130 milliseconds, the time delay and gatecircuit thus represent a lockout for the short light 60 such that theshort light 60 will not be energized unless it is energized prior to the130 millisecond transition time. With the foregoing, it can be seen thatthis inventiontwill attempt to cure, or will cure, a fault in the sensorand the output winding which lights the fire warning light 22.

Since the foregoing is a description of a generally conventionalmagnetic amplifier, no further description thereof is deemed necessaryherein. It should be made clear, however, that for the purposes of thepresent invention, the magnetic amplifier is utilized to explain theoperation of the fire warning system since it is one type of impedancedetector utilized to detect the decrease in resistance in the sensorsdue to heat and to energize a fire warning light. Other systems may havesome other means for obtaining a reflected impedance and it is to beunderstood that the present invention will be compatible with suchsystems once the description of the operation of the invention and itsoperation as well as the other refinements herein disclosed areunderstood by those skilled in the art.

Attention is now directed in particular to the clearing relay 42, whichconnects the DC clearing supply voltage 40, to apply a pulse of highvoltage to the sensor 10 when the sensing means is cycled in response toa decrease in resistance in the sensor 10.

Clearing relay 42 is a double pole, double throw, relay and shown inthe'position of normal operation in the existing system, i.e., in aposition in which a source of DC voltage from supply 40 is disconnectedfrom the sensing line 20, but with one swinger 10 6 closed on contact108 closing the sensing line 20 between magnetic amplifier 18 and sensor10. Contact 110 connected to the DC voltage supply 40 is shown open but,upon actuation of the relay 42 to throw swingers 106 and 112, DC powersupply 40 will apply a pulse of DC voltage through swinger 112, contact110, line 116, and swinger 106 which closed onto previously open contact118. The other contact 120 for swinger 112, shown closed, 'is connectedthrough line 116 to the other swinger 106 and to sensing line 20 toinsure operativeness of the existing fire warning system. That is tosay, if the clearing relay 42 becomes inoperative, as for example, ifone of the swingers should stick in either of its positions and thus benon-responsive to the relay coil 124, sensor 10 will still be coupledinto the magnetic amplifier 18. This is a refinement in this inventionto make it completely compatible with the present fire Warning system.

Thus, upon actuation of the relay coil 124 to connect contact 112 toline 20, a pulse of DC voltage is applied to the sensor 10 from clearingsupply 40. This supply 40 comprises capacitor 130 (1 mid, 150 v.)supplied with energy from terminal 132 and diode rectifier 134 connectedto the ships AC supply (115 v., 400 c.p.s.) through filter capacitor 136and current limiting resistor 138.

The clearing relay 42 which supplies the fault clearing pulse of DCcurrent from clearing supply 40 to sensor 10 is monitored by the sensingmeans which comprises aforementioned amplifier 50, rectifier 52, filter'54, clearing trigger circuit 56 and clearing cycle limiter 58 whichwill now be described.

The level of AC voltage (3.5 volts) from the control winding 90 of themagnetic amplifier 18 is monitored by the sensing means at relay contact108 by the capacitance coupled amplifier 50.

Amplifier 50 is of the emitter follower type selected to have highimpedance and for high current amplification and comprises a transistor146 whose emitter is coupled to an output transformer 148 and which isprovided with suitable collector and base resistance coupled in theconventional manner. The output of the transformer 148 is fed into arectifier and voltage doubler network comprising diodes 160, 162 andcapacitor 164 to rectify the output level of voltage to about 25 v.which is then filtered before being coupled into the clearing triggercircuit 56 and into the short trigger circuit 64. The filter circuit forthe clearing trigger 56 is coupled to the output of the voltage-doublerrectifier network 52 at tap 170 and com-prises a pair of resistors 172and 174 and capacitor 176 which is coupled to an isolation resistor 178of the clearing trigger 56.

The clearing trigger circuit 56 as disclosed herein is a Schmitt triggerof conventional design having a pair of NPN transistors 190 and 192. Theoperation of this type of trigger, a bistable one-shot multi-vibrator,depends on the amplitude of the voltage from the isolation resistor 178coupled to the base of the normally on transistor 190 (operated atsaturation). The collector of the transistor 190 is connected to theships supply of DC voltage (+28 V. DC.) at tap 194 and to the base ofthe second transistor 192 through suitable collector and base resistors.The collector of the second transistor is in turn also connected to theships supply of DC voltage through a suitable collector resistor and isalso connected to line 200 which couples the clearing trigger 56 to theclearing cycle limiter 58, the time delay circuit 74 and to the shortdetector 68.

The clearing cycle limiter 58 comprises a limiting capacitor 210 and acapacitor discharge resistor 212 in parallel therewith, both of whichare in parallel with the output of transistor 192 of the clearingtrigger. Capacitor discharge resistor 212 permits the capacitor 210which is only partially charged by one pulse from the clearing triggercircuit 58 to discharge, provided the clearing trigger circuit does notpulse the capacitor 210 again within a predetermined time.

With the actuation of the clearing relay 42, applying the clearing pulsefrom capacitor to the sensor 10 to cure the fault, the sensing line 20returns to itsnormal AC voltage (3.5 v.) since the fault short in thesensor is then cut out. This rise in voltage in sensing line 20 isreflected in the clearing trigger 56 in a decrease to 25- 26+ in voltagebias on the base of transistor 190 which again returns to saturation andthe second transistor 192 is returned to cut off, its original state. Ifa clearing of the fault does not occur, the clearing relay 42 will againbe actuated by the clearing trigger 56 because of the increase involtage again appearing at the base of the on transistor 190 to flip thetrigger and to again discharge another pulse of DC from power supply 40through the sensor. This cycling of the relay would go on, ad infinitum,without the limiting capacitor 210 in the circuit which limits thecycling of the relay 42. It can be understood that one pulse fromtrigger circuit 56 only partially charges limiting capacitor 210 but thesize of the latter and the capacitor discharge resistor 212 are selectedso that after a limited number of charges, i.e., three, the capacitor isfully charged and will not pass a pulse from the clearing trigger 56 tothe relay coil 124 to actuate the relay. Once fully charged, theresistor 212 will discharge the capacitor 210 but only after either thefire warning light 22 or the short light is energized. Experience hasshown however that upon the discharge of the high voltage to the sensor10 three times, it is an indication that a true fire or permanent shortexists in the system and, consequently, either the fire warning light 22or the short light 60 will be energized in reaction to the continueddecrease in resistance in the system. The resistance 212 has stillanother advantage and that is, it acts to place the invention inoperation substantially immediately after the pilot test switch isactuated by discharging the condensor 210 only partially charged at thattime.

More specifically, if the pilot switch 32 were actuated, the relay 42would respond three times in accordance with the preselected setting ofthe clearing cycle limiter. Since the capacitor discharge resistor 212allows the charge on the capacitor 210 to bleed 01f, if the pilot testbutton is again immediately depressed, the system will again operatethree times. This is a safety feature in that it could be coincidentalthat the pilot test button is depressed a the same time either a shortor a fire is occurring, in which case, the pressing of the ,pilot testbutton does not cause any delay in the actuation of the system.

Turning now to the short indicator light 60 and the means for energizingand controlling the same, which as previously mentioned comprises theaforementioned am plifier 50, rectifier and voltage doubler 52, a filter62 coupled to the output of the rectifier, a short trigger circuit 64, agate circuit 66 which is coupled to a time delay circuit 74 and to theshort detector 68. The short detector 68 is in turn coupled to a relaydriver 70 to operate the relay switch 72.

It can be seen that the short trigger 64 is coupled to the rectifier andvoltage doubler 52 at tap through the second filter 62, which issubstantially identical to the other filter 54 coupled to the input ofthe clearing trigger 56 and for that reason no further description ofthis filter is deemed necessary. The short trigger 64 is also identicalto the clearing trigger 56 and no further description of the elementsthereof is deemed necessary except that it should be noted that theoutput of the short trigger 64 is coupled to the collector of the firststage transistor 220 (as distinguished from the clearing trigger whoseoutput is coupled to the second stage transistor collector). The outputof this first stage is then coupled into the gate circuit 66. Gatecircuit 66 is an AND or coincidence gate of conventional design, andcomprises tWo diodes 230 and 232; diode 230 being connected to theoutput of the short trigger 64 and diode 232 being connected to theoutput of the time delay circuit 74. Characteristic of this type ofgate, both inputs must be brought to a predetermined level of voltagebefore an output is provided from the gate. The outputs of the twodiodes 230 and 232 are connected to a third or clamping diode 234 whichin turn is connected to the input of the short detector 68.

The short detector 68 is a conventional bistable multivibratorcomprising a pair of common emitter coupled NPN transistors 240 and 242whose collectors and bases are respectively coupled together and whichis provided with suitable base and load resistors. In this circuit theoutput from the clamping diode 234 is coupled to the base of the firststage transistor 240. The first stage transistor 240 is biased to cutoff or off, and the second transistor 242 is biased to saturation or on.

The short detector 68 is operated only when the two transistors 240 and242 are simultaneously gated by the short trigger 64 and time delay 74.The short detector 68 is maintained in an on condition by the shorttrigger 64 via the steering diode network, resistor 244 and diode 246;however, this path through this network does not initiate the shortdetector operation. On the other hand, if the short detector isoperated, it pulses this relay driver 70 which is a conventionalDarlington amplifier comprising a pair of transistors 250 and 252; thebase of the latter being coupled to the emitter of the first stage andthe collector of the second stage being coupled to relay 72.

As mentioned before, the output of the time delay circuit 74 is coupledto the gate circuit 66 as also is the short trigger output.

It should also be clear that the time delay 74 also operates to permitthe magnetic amplifier to indicate a fire from the energization of thefire warning light 22 because it will prevent the short detector fromenergizing the relay driver and hence the short light 60 unless it doesso within a predetermined time (i.e., 100 milliseconds or as required).As mentioned before, a short in the system will be reflected by animmediate drop in resistance where as the drop in resistance due to afire takes from 300 milliseconds to minutes. At this latter time, thetime delay will prevent the operation of the short trigger by reason ofthe bias applied to the output diode 232 in the gate 66.

The time delay 74 comprises a monostable multivibrator circuitcomprising a pair of common emitter coupled transistors 260 and 262. Thecollector of the first stage is coupled to a time constant networkcomprising capacitor 264, fixed resistance 266, and a variableresistance 268. The capacitor 264 is also coupled to a diode 270 whichin turn is connected to the base of the second stage 262. As thusarranged, when the base of the first stage receives a pulse from theoutput from the clearing trigger, the normally off transistor 260 isrendered conducting causing a spike in the collector. circuit which isreflected into the time constant network. With the first stage 260operating the second stage is rendered nonconducting causing a voltageincrease in the collector circuit of the second stage 262 which in turnis reflected into the gate circuit 66. This voltage increase, however,is delayed by the time constant circuit the time of which is fixed bythe variable resistor 268.

While the above invention has been described in connection with amagnetic amplifier, it is to be understood that this is but one exampleof the type of impedance which is used in a typical fire warning system.Other impedance devices may be used, for example, a transistor circuitto pulse a DC current through the sensor 10 in which case the inventionmay be modified to match the circuitry by one skilled in the art havingunderstood the present invention. For example, parts of the presentinvention may be moduled apart from other parts (for example theamplifier and rectifier in one module and the remainder in othermodules), to make the invention readily interchangeable with differentfire warning systerns; it being important to understand that the majoraspect of this invention is in the discovery that the pulsing of a DCvoltage (or a steep wave front AC) through the sensor clears the shortsdue to faults and thus reduce the high incident of false alarms and thatthe other modifications and refinements thereof are to make the systemcompatible with existing systems as well as to give the crew moreinformation about the condition of the system than heretofore availableto them. In connection with this latter, for example, it is within thepurview of this invention to include a modification to the pilot testswitch 32 to make it a three position switch with different resistancelevels at each position which match the resistance levels prescribed inthe graph (FIGURE 4) in order to test the reaction of the circuitry ateach level.

Although the invention has been described with reference to a particularembodiment thereof, it is to beappreciated that many modifications arepossible without departing from the scope and spirit of the invention.

What is claimed is:

1. An improvement for use in a fire warning system having a heat sensorand a detector to sense impedance change and to actuate a warningindicator at predetermined impedance level due to an increase intemperature of the heat sensor, comprising:

means coupled to said system and responsive to a decrease in impedanceacross the sensor for applying a plurality of electrical pulses to saidheat sensor for clearing said sensor of any fault occurring thereinwhich is reflected in said detector by a decrease in impedance.

2. An improvement for use in a fire warning system having a heat sensorand a detector to sense impedance change and to actuate a warningindicator at predetermined impedance level due to an increase intemperature of the heat sensor, comprising:

means coupled to said system for applying a plurality of electricalpulses to said heat sensor for clearing said sensor of any faultoccurring therein which is reflected in said detector by a decrease inimpedance, and

means for sensing and for actuating said pulsing means upon apredetermined impedance decrease.

3. An improvement for use in a fire warning system having a heat sensorand a detector to sense impedance changes and to actuate a warningindicator at predetermined impedance level due to an increase intemperature of the heat sensor, comprising:

pulsing means coupled to said system for applying a plurality ofelectrical pulses to said heat sensor for clearing said sensor of anyfault occurring therein which is reflected in said detector by adecrease in impedance,

means for sensing and actuating said pulsing means upon a predeterminedimpedance decrease, and means for limiting the actuation of said pulsingmeans to a preselected number of pulses.

4. An improvement for use in a fire warning system having a heat sensorand a detector to sense impedance changes and to actuate a warningindicator at predetermined impedance level due to an increase intemperature of the heat sensor comprising:

means coupled to said system and responsive to a decrease in impedanceacross said sensor for applying a plurality of electrical pulses to saidheat sensor for clearing said sensor of any fault occurring thereinwhich is reflected in said detector by a decrease in impedance, and

means for discriminating between a decrease in impedance due to a shortin the system and a decrease in impedance due to an increase intemperature of said heat sensor.

5. The improvement claimed in claim 4 further including means foractuating a short warning indicator in the event a short is detected bysaid discriminating means.

6. The improvement claimed in claim 5 further including means forpreventing operation of said short warning indicator if the heat warningindicator is actuated and for preventing operation of said heat warningindicator if said short warning indicator is actuated.

7. In combnation with a fire warning system having at least one heatsensor and a detector to apply electrical energy to said heat sensor todetect a decrease in resistance in said system due to an increase intemperature of said heat sensor and to actuate a fire warning indicatorwhen said resistance drops to a predetermined low level, the improvementcomprising:

means responsive to a decrease in impedance across said sensor forclearing said heat sensor of any short circuits which may occur in saidsensor due to factors other than heating of said sensor and which wouldotherwise be detected by said detector as an increase in temperature insaid sensor and thus actuate said warning indicator falsely.

8. In combination with a fire warning system having at least one heatsensor and a detector to apply electrical energy to said heat sensor todetect a decrease in resistance in said heat sensor due to an increasein temperature of said heat sensor and to actuate a fire warningindicator when said resistance drops to a predetermined low level, theimprovement comprising:

and means responsive to a decrease in impedance across said heat sensorfor clearing said heat sensor of any short circuits which may occur insaid sensor due to factors other than heating of said sensor and whichwould otherwise be detected by said detector as an increase intemperature in said sensor and thus actuate said warning indicatorfalsely, said means comprising means for applying said sensor with oneor more pulses of electrical energy.

9. In combination with a fire warning system having at least one heatsensor and a detector to apply electrical energy to said heat sensor todetect a decrease in resistance in said system due to an increase intemperature of said heat sensor and to actuate a fire warning indicatorwhen said resistance drops to a predetermined low level, the improvementcomprising:

means responsive to a decrease in impedance across said heat sensor forclearing said heat sensor of any short circuits which may occur in saidsensor due to factors other than heating of said sensor and which wouldotherwise be detected by said detector as an increase in temperature insaid sensor and thus actuate said warning indicator falsely, and

means for actuating a short warning indicator in the event theresistance in said system continues to drop to a resistance level at orbelow the aforesaid predetermined resistance level but reaches saidlevel within a preselected time.

10. In combination with a fire warning system having at least one heatsensor and a detector to apply electrical energy to said heat sensor todetect a decrease in resistance in said system due to an increase intemperature of said heat sensor and to actuate a fire warning indicatorwhen said resistance drops to a predetermined low level, the improvementcomprising:

means for clearing said heat sensor of any short circuits which mayoccur in said sensor due to factors other than heating of said sensorand which would otherwise be detected by said detector as an increase intemperature in said sensor and thus actuate said warning indicatorfalsely,

means responsive to a decrease in impedance across said heat sensor foractuating said clearing means,

means for limiting the number of times said actuating means operates asthe resistance in said system continues to drop,

means for actuating a short warning indicator if the resistance in thesystem drops to a predetermined level within a predetermined time, and

means for locking out said short warning indicator actuator after saidpredetermined time to enable said detector to actuate said fire warningindicator.

11. In combination with a fire warning system having at least one heatsensor and a detector to apply electrical energy to said heat sensor todetect a decrease in resistance in said heat sensor due to an increasein temperature of said heat sensor and to actuate a fire warning 0indicator when said resistance drops to a predetermined low level, theimprovement comprising:

a source of voltage for clearing said heat sensor of any short circuitswhich may occur in said sensor due to factors other than heating of saidsensor and which would otherwise be detected by said detector as anincrease in temperature in said sensor and thus actuate said firewarning indicator falsely, and

means responsive to a decrease in resistance across said heat sensor forintermittently applying a source of voltage to said heat sensor.

12. A fire warning system including:

a heat sensor comprising an outer tubular shell of conductive material,an inner coaxial conductor, and a heat sensitive material between saidinner center conductive material, an inner coaxial conductor, andmaterial having high electrical resistance and insulat ing said centerconductor from said outer conductor at a predetermined low temperaturelevel and having a low resistance and shorting said center conductorfrom said outer conductor when said heat sensitive material reaches apredetermined higher temperature level,

means coupled to said center conductor and said outer shell of said heatsensor for applying electrical energy across said heat sensitivematerial,

means for sensing the drop in resistance of said heat sensitive materialto a predetermined low level and for actuating a warning indicator whensaid resistance reaches said low level to warn of an increase in heat atsaid sensor, said heat sensitive material being subject to faults whichalso short the center conductor to the outer shell and thus when sensedby said sensing means would also actuate said warning indicator butwhich would be interpreted roughly as an increase in heat at saidsensor, and

means for discriminating and clearing said heat sensor of faults beforesaid warning indicator is actuated by said detector.

13. The fire warning system claimed in claim 12 wherein said lastmentioned means comprises:

a source of electrical energy, and

means for intermittently coupling said source of electrical energyacross said heat sensitive material to cure said heat sensitive materialof faults.

14. The fire warning system as claimed in claim 13 wherein said meansfor intermittently coupling said source of energy to said centerconductor comprises a clearing trigger, circuit and a switching relay,said clearing trigger circuit actuating said relay so that the latterwill connect said source of energy to said center conductor.

15. The fire warning system as claimed in claim 14 wherein means areprovided for preventing further actuation of said relay after apredetermined number of times.

16. The fire warning system claimed in claim 15 further including meansfor discriminating between further re- 7 l5 sistance drop in said systemdue to a'short other than a fault and a fire.

17..The fire warning system as claimed in claim 16 wherein said meansfor discriminating between a short due other than to a fault comprises:

a short trigger circuit connected to said system, a time delay circuit,a coincidence gating circuit, and a bistable multivibrator, the outputof said trigger circuit being connected to said time delay and theoutput of said short trigger circuit being connected to said gatingcircuit and to said bistablemultivibrator so that when said clearingtrigger and short trigger are triggered, said time I delay preventsoperation of said bistablerfiult ivibrator unlessthe resistance drop insaid system falls to a predetermined level within a predetermined time.

References Cited UNITED STATES PATENTSY, I

D. L. TRAFTON, Assistant Examiner.

